Short Circuits, Ground Faults and Ground Fault Circuit Interrupter

Posted by PITHOCRATES - April 9th, 2014

Technology 101

AC Power uses Reciprocating Currents to produce Rotating Electromagnetic Fields

There is a police crime lab television show that can solve a crime from a single fiber.  Many crime lab shows, actually.  Where they use high-tech science and music montages to solve many a crime.  Which is great if you DVR’d the shows as you can fast forward through them.  And save some time.  In one of these shows the writers goofed, though.  Because they didn’t understand the science behind the technology.

Someone murdered a construction worker by sabotaging a power cord.  By cutting off the grounding (or third) prong.  The fake crime scene person said this disabled the ground fault circuit interrupter (GFCI) device in the GFCI receptacle.  Leaving the user of the cord unprotected from ground faults.  So when said worker gripped the drill motor’s metallic case while standing in water and squeezed the trigger he got electrocuted.  And when the investigator saw that someone had cut off the grounding prong of the cord he said there was no way for the GFCI to work.  Which is, of course, wrong.  For the grounding prong has little to do with tripping the GFCI mechanism in a receptacle.

If you look at an electrical outlet you will see three holes.  Two vertical slots and one sort of round one.  Inside of these holes are pieces of metal that connect to wiring that runs back to the electric panel in your house.  One of the slots is the ‘hot’ circuit.  The other slot is the ‘neutral’ circuit.  And the third slot is the ‘ground’ circuit.  Now alternating current (AC) goes back and forth in the wiring.  It will come out of the hot and go into the neutral.  Then it will reverse course and come out of the neutral and go into the hot.  Think of a reciprocating engine where pistons go up and down to produce rotary motion.  AC current does the same to produce rotating electromagnetic fields in an electric motor.

The Current in our Electric Panels wants to Run to Ground

If the current can come out of both the hot and the neutral why aren’t both of these slotted holes hots?  Or both neutrals?  Good question.  The secondary winding on the pole-mounted transformer feeding your house has three wires coming from it.  The secondary is a very long wire wrapped many times around a core.  If you measure the voltage at both ends of this coil of wire you will get 240 volts.  They also attach a third wire to this coil of wire.  Right in the center of the coil.  So if you measure the voltage from this ‘center tap’ to one of the other two wires you will be measuring the voltage across half of the windings.  And get half of the voltage.  120 volts.

These are the three wires they bring into your house and terminate to your electric panel.  The center tap and the two wires coming off the ends of the secondary winding.  They attach each of the two ‘end wires’ to a hot bus bar in the panel.  And attach the center tap to the neutral bus.  They also connect the ground bus to the neutral bus.  A 1-pole circuit breaker attaches to one of the two hot bus bars.  Current travels along a wire attached to the breaker, runs through the house wiring, goes through the electrical load and back to the panel to the neutral bus.  So this back and forth current comes from the 120 voltage produced over half of the secondary coil of wire in the transformer.  Where as a 2-pole breaker attaches to both hot bus bars.  Current travels along a wire attached to one pole of the breaker, runs through the house wiring, through the electric load and back to the panel.  But instead of going to the neutral bus bar it goes to the other pole of the 2-pole breaker and to the other hot bus bar.  So this back and forth current comes from the 240 voltage produced across the whole secondary coil in the transformer.

Current wants to run to ground.  It’s why lightning hits trees.  Because trees are grounded.  The current in our electric panels wants to run to ground, too.  Which we only let it do after it does some work for us.  When we plug a cord into an electric outlet we are bringing the hot and neutral closer together.  Like when we plug in our refrigerator.   When the temperature falls a switch closes completing the circuit between hot and neutral through the compressor in the refrigerator.  So the current can run to ground.  Which is actually a back and forth motion through the conductors to create a rotating electromagnetic field in the compressor.  Which runs back and forth between one of the hot bus bars and the neutral bus bar in the panel.

Ground Faults don’t trip Circuit Breakers when finding an Alternate Path to Ground

When we stand on the ground we are grounded.  We are physically in contact with the ground.  We can lie on the ground and not get an electric shock.  Despite all current wanting to run to ground.  So if all current is running to ground why don’t we get a shock when we contact the ground?  Because we are at the same potential as the ground.  And no current flows between objects at the same potential (i.e., voltage).  This is the reason why we have a ground prong on our cords.  And why we install a bonding jumper between the neutral bus and the ground bus in our panels.  So that everything but the hot bus bars is at the same potential.  So no current flows through anything UNLESS that something is also connected to a wire running back to a hot bus in the panel.

Of course, if there is lightning outside we don’t want to be the tallest object out there.  For that lightning will find us to complete its path to ground.  Just as electricity will inside our house.  This is the purpose of the grounding prong on cords.  And why we ground all metallic components of things we plug into an electric outlet.  So if a hot wire comes loose inside of that thing and comes into contact with the metal case it will create a short circuit to ground for that current.  The current will be so great as it flows with no resistance that it will exceed the trip rating of the circuit breaker.  And open the breaker.  De-energizing everything in contact with that loose hot wire.  Eliminating an electric shock hazard.  For example, you could have a fluorescent light with a metal reflector in your basement.  It could have a loose hot wire that energizes the full metallic exterior of that light.  If you were working in the ceiling and had one hand on a cold water pipe when you came into contract with that light you would get a nasty electric shock.  But if it was grounded properly the breaker would trip before anyone could suffer an electric shock.

Ground faults are a different danger.  Because they don’t trip the circuit breaker in the panel.  Why?  Because it’s not a short circuit to ground.  But current taking a different path to ground.  That doesn’t end inside the electric panel.  For example, if you’re using a hair dryer in the bathroom you may come into contact with water and cold water piping.  Things that can conduct electricity to ground.  And if you are in contact with these alternate paths to ground some of that current in the hot wire will not equal the current in the neutral wire.  Because that back and forth current will be going in and out of the hot bus.  And in and out of a combination of the neutral bus and that alternate path to ground through you.  Electrocuting you.  But because of your body’s resistance the current flow through you will not exceed the breaker rating.  Allowing the current to keep flowing through you.  Perhaps even killing you.  This is why we have GFCI receptacles in our bathrooms, kitchens and anywhere else there may be an alternate path to ground.

So how does a GFCI work?  When current flows through a wire it creates an electromagnetic field around the wire.  If you’re looking into the wire as it runs away from you the field will be clockwise when the current is going away from you.  And counter clockwise when coming towards you.  In an AC circuit there are two conductors with current flow.  And at all times the currents are equal and run in opposite directions.  Cancelling those electromagnetic fields.  Unless there is a ground fault.  And if there is one the current in the neutral will decrease by the amount running to ground.  And the electromagnetic field in the neutral conductor will not cancel out the electromagnetic field in the hot conductor.  The GFCI will sense this and open the circuit.  Stopping all current flow.  Even if the ground prong was cut off.

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Electric Power, Alternating Current, Transformers, Magnetic Flux, Turns Ratio, Electric Panel and Circuit Breakers

Posted by PITHOCRATES - February 6th, 2013

Technology 101

AC Power is Superior to DC Power because it can Travel Farther and it Works with Transformers

Thanks to Nikola Tesla and his alternating current electric power we live in the world we have today.  The first electric power was direct current.  The stuff that Thomas Edison gave us.  But it had some serious drawbacks.  You needed a generator for each voltage you used.  The low-voltage of telephone systems would need a generator.  The voltage we used in our homes would need another generator.  And the higher voltages we used in our factories and businesses would need another generator.  Requiring a lot of power cables to hang from power poles along our streets.  Almost enough to block out the sun.

Another drawback is that direct currents travel a long way.  And spend a lot of time moving through wires.  Generating heat.  And dropping some power along the way due to the resistance in the wires.  Greatly minimizing the area a power plant can provide power to.  Requiring many power plants in our cities and suburbs.  Just imagine having three coal-fired power plants around your neighborhood.  The logistics and costs were just prohibitive for a modern electric world.  Which is why Thomas Edison lost the War of Currents to Nikola Tesla.

So why is alternating current (AC) superior to direct current (DC) for electric power?  AC is more like a reciprocating motion in an internal combustion engine or a steam locomotive.  Where short up & down and back & forth motion is converted into rotation motion.  Alternating current travels short distances back and forth in the power cables.  Because they travel shorter distances in the wires they lose less power in power transmission.  In fact, AC power lines can travel great distances.  Allowing power plants tucked away in the middle of nowhere power large geographic areas.  But there is another thing that makes AC power superior to DC power.  Transformers.

The Voltage induced onto the Secondary Windings is the Primary Voltage multiplied by the Turns Ratio

When an alternating current flows through a coiled wire it produces an alternating magnetic flux.  Magnetic flux is a measure of the strength and concentration of the magnetic field created by that current.  When this flux passes through another coiled wire it induces a voltage on that coil.  This is a transformer.  A primary and secondary winding where an alternating current applied on the primary winding induces a voltage on the secondary winding.  Allowing you to step up or step down a voltage.  Allowing one generator to produce one voltage.  While transformers throughout the power distribution network can produce the many voltages needed for doorbells, electrical outlets in our homes and the equipment in our factories and businesses.  And any other voltage for any other need.

We accomplish this remarkable feat by varying the number of turns in the windings.  If the number of turns is equal in the primary and the secondary windings then so is the voltage.  If the number of turns in the primary windings is greater than the number of turns in the secondary windings the transformer steps down the voltage.  If the number of turns in the secondary windings is greater than the number of turns in the primary windings the transformer steps up the voltage.  To determine the voltage induced onto the secondary windings we divide the secondary turns by the primary turns.  Giving us the turns ratio.  Multiplying the turns ratio by the voltage applied to the primary windings gives us the voltage on the secondary windings.  (Approximately.  There are some losses.  But for the sake of discussion assume ideal conditions.)

If the turns ratio is 20:1 it means the number of turns on the primary windings is twenty times the turns on the secondary windings.  Which means the voltage on the primary windings will be twenty times the voltage on the secondary windings.  Making this a step-down transformer.  So if you connected 4800 volts to the primary windings the voltage across the secondary windings will be 240 volts (4800/20).  If you attached a wire to the center of the secondary coil you can get both a 20:1 turns ratio and a 40:1 turns ratio.  If you measure a voltage across the entire secondary windings you will get 240 volts.  If you measure from the center of the secondary and either end of the secondary windings you will get 120 volts.

The Power Lines running to your House are Two Insulated Phase Conductors and a Bare Neutral Conductor

This is a common transformer you’ll see atop a pole in your backyard.  Where it is common to have 4800-volt power lines running at the top of poles running between houses.  On some of these poles you will see a transformer mounted below these 4800-volt lines.  The primary windings of these transformers connect to the 4800-volt lines.  And three wires from the secondary windings connect to wires running across these poles below the transformers.  Two of these wires (phase conductors) connect to either end of the secondary windings.  Providing 240 volts.  The third wire attaches to the center of the secondary windings (the neutral conductor).  We get 120 volts between a phase conductor and the neutral conductor.

The power lines running to your house are three conductors twisted together in a triplex cable.  Two insulated phase conductors.  And a bare neutral conductor.  These enter your house and terminate in an electric panel.  The two phase conductors connect to two bus bars inside the panel.  The neutral conductor connects to a neutral bus inside the panel.  Each bus feeds circuit breaker positions on both sides of the panel.  The circuit breaker positions going down the left side of the panel alternate between the two buss bars.  Ditto for the circuit breaker positions on the right side.

A single-pole circuit breaker attaches to one of the bus bars.  Then a wire from the circuit breaker and a wire from the neutral bus leave the panel and terminate at an electrical load.  Providing 120 volts to things like wall receptacles where you plug things into.  And your lighting.  A 2-pole circuit breaker attaches to both bus bars.  Then two wires from the circuit breaker leave the panel and attach to an electrical load.  Providing 240 volts to things like an electric stove or an air conditioner.  Then a reciprocating (push-pull) alternating current runs through these electric loads.  Driven by the push-pull between the two bus bars.  And between a bus bar and the neutral bus.  Which is driven by the push-pull between the conductors of the triplex cable.  Driven by the push pull of secondary windings in the transformer.  Driven by the push-pull of the primary windings.  Driven by the push-pull in the primary cables connected to the primary windings.  And all the way back to the push-pull of the electric generator.  All made possible thanks to Nikola Tesla.  And his alternating current electric power.

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